The invention relates to devices and methods for attenuating acoustic and/or thermoacoustic oscillations in combustors, in particular in combustors of gas turbines.
Combustors today are designed primarily with an eye toward the lowest possible formation of noxious substances, and thus with the lowest possible discharge of noxious substances during the operation of the combustor. Significant noxious substances formed during combustion are nitrogen oxides which, depending upon the atmospheric altitude in which they are discharged, can cause either a decrease or increase in ozone. Nitrogen oxides (NOx) form at very high temperatures. Such high temperatures occur during combustion with, in particular, a slight excess of air, i.e. a rich combustion. Such conditions exist, for example, in case of an insufficient atomization and gasification of a liquid fuel in the immediate area around fuel droplets. In order to prevent the formation of nitrogen oxides, today""s combustors are mostly designed as premixing combustors. In this case, the fuel, which is mostly gaseous in stationary gas turbines, is first mixed in a premixing device with air prior to the actual combustion. The premixing device often consists of one or more burners, such as are described, for example, in publication DE 43 04213 A1. Furthermore, the admixture of secondary air during the combustion process is now either absent or almost absent in modern combustors. The air supplied for combustion therefore flows completely or almost completely through one or more burners into the combustion chamber at its inlet. This causes a highly homogeneous gas/air mixture to form in the combustion chamber. Thus, a local fuel/air mixture that is too rich can be substantially prevented. As a result, the nitrogen oxide formation can be reduced.
The construction of such a so-called Low-NOx combustor differs from standard combustors in particular in the air supply. As was already mentioned, no secondary air, or almost none, is mixed in with the internal flow of the combustion chamber downstream from the combustion chamber inlet. In standard combustors, secondary air is supplied via bores in the combustion chamber wall, in particular for cooling the wall housing of the internal combustion chamber flow. The secondary air flowing into the combustion chamber also resulted in a stabilization of the combustion flow. In addition to an aerodynamic stabilization of the flame, the inflowing secondary air also resulted in a strong acoustic attenuation inside the combustor. Wall pressure fluctuations in the combustor undergo a particularly strong attenuation due to the incoming secondary air flow, particularly if the secondary air mass flux is large and the entry speed is low. Because of this high acoustic pressure level, the combustor in return had a high attenuation capacity with respect to the acoustic and/or thermoacoustic oscillations of the combustor which were attenuated by dissipation. The absence of a secondary air supply in the combustion flow in modern combustors, in contrast, has led to a low acoustic attenuation of the combustors. Acoustic and/or thermoacoustic oscillations occur in combustors as a result of different causes. Inhomogeneous temperature distributions in the combustion flow when passing through the turbine result, for example, in inhomogeneous pressure and therefore thermoacoustic oscillations because of a spatially or temporarily inhomogeneous enthalpy conversion. These oscillations, in principle, cannot be prevented. In the presence of a too low attenuation and in relation to the acoustic behavior of the combustor, for example of the natural frequencies, these oscillations may, however, result in undesired high pressure amplitudes. In addition to a high mechanical strain of the combustor due to the pressure change amplitudes, this results in increased emissions of noxious substances as a result of inhomogeneous combustion and, in the extreme case, in an extinguishing of the flame.
To attenuate such acoustic and/or thermoacoustic oscillations, Helmholtz resonators, as described in the publication by J. J. Keller and E. Zauner, xe2x80x9cOn The Use Of Helmholtz Resonators As Sound Attenuatorsxe2x80x9d, Zangew Math Phys 46, 1995, p. 297-327, were used in the past. These Helmholtz resonators are hereby connected at least on the inlet side to the combustion chamber. But Helmholtz resonators function only in a narrow frequency band around a base frequency. This does not therefore provide any broad-band attenuation of different oscillation frequencies.
The invention therefore is based on the objective of effectively attenuating acoustic and/or thermoacoustic oscillations in a combustor of a turbo machine, in particular a gas turbine, over the largest possible frequency range.
According to the invention this objective is realized in that the combustor has at least one fluid supply device and one combustion chamber, and that the combustion chamber furthermore has at least one recirculation opening for attenuating acoustic and/or thermoacoustic oscillations. The recirculation opening provides a local pressure compensation for acoustic and/or thermoacoustic oscillations, resulting in a destructive interference of acoustic waves and their reflections. Depending on the pressure conditions in front of and behind the recirculation opening, an inflow or outflow of fluid through the recirculation opening occurs with acoustic and/or thermoacoustic oscillations. Naturally, a perfect pressure compensation would require that the flow speed would just disappear. It is useful that the recirculation opening merges into the fluid inflow to the combustion chamber, i.e. it flows in a useful manner into the fluid supply device, But the recirculation opening also may additionally merge with another volume. If it merges with the fluid inflow, the fluid flowing from the combustion chamber is transported along with the fluid flowing into the combustion chamber. This results in a reentry of the flow into the combustor, and thus in a recirculation of the fluid flowing out of the combustion chamber. But given the appropriate pressure conditions, the fluid from the fluid infeed can also flow through the recirculation opening into the combustion chamber. Without restricting either of the two possible flow directions through the recirculation opening, however, as a rule, the present invention is concerned primarily with the outflow of fluid from the combustion chamber. With a suitable, preferable design of the combustor, a useful, primarily very small outflow of fluid through the recirculation opening from the combustion chamber occurs. Furthermore, while not limiting the general application, only the recirculation of the fluid will be considered for reasons of simplification. It was found that acoustic and/or thermoacoustic oscillations of the combustor are attenuated in a sustained manner as a result of the pressure compensation near the recirculation openings.
At least part of the fluid supply device preferably extends so as to immediately adjoin the outside of the combustion chamber wall. Along with the supply of a fluid, in most cases air, to the combustion chamber of the combustor, this arrangement of the fluid supply device causes the combustion chamber wall on the outside of the combustion chamber to be cooled convectively. The fluid in the fluid supply device in this case therefore flows in reverse direction to the flow in the combustion chamber. It is useful that the fluid supply device merges into an antechamber, and from there into the combustion chamber. It is hereby desired that the most homogeneous flow status of the fluid that is possible develops in this antechamber. The flow status of the fluid relates to the static pressure, temperature, and flow speed of the fluid. An inhomogeneous flow status would lead to an inhomogeneous inflow into the combustion chamber of the combustor, and finally to an inhomogeneous combustion occurring in the combustion chamber. A more simple version of the combustor may be formed which eliminates this antechamber. It is useful that the fluid flows completely or almost completely on the entry side, preferably via a front panel located on the entry side, into the combustion chamber. The combustion chamber is frequently constructed in a circular or annular shape, whereby the front panel terminates the combustion chamber on the entry side. Because of the complete or almost complete infeeding of the fluid into the combustion chamber via the front panel, the combustion taking place in the combustion chamber from the outset has available a fluid quantity that is sufficient for a combustion process with low quantities of noxious substances. For the purpose of a low-noxious combustion, it is furthermore advantageous that the combustor is constructed as a premixing combustor with a premixing device. A premixing of the mostly gaseous fuel with air takes place in the premixing device. The premixing device which is preferably constructed as a burner is advantageously located in front of the combustion chamber and preferably merges into the combustion chamber at the level of the front panel.
The recirculation opening is preferably located in the front part of the combustion chamber on the combustion chamber wall and/or the front panel. The recirculation opening in the front part of the combustion chamber causes the acoustic oscillation in the area of a primary combustion zone to have a pressure node. But since the pressure oscillation amplitude in the primary combustion zone is maintained near zero, it is also not possible for any strong sound stimulation to occur according to the xe2x80x9cRayleigh criterionxe2x80x9d. The combustion chamber therefore presents in its front part an at least partially open oscillation chamber.
In an advantageous arrangement, the recirculation opening is connected with the fluid supply device and/or the antechamber. If as a result of an acoustic and/or thermoacoustic oscillation fluid flows through the recirculation opening from the combustion chamber, this fluid thus flows into the fluid supply device and/or the antechamber. From there, the fluid flowing from the combustion chamber then flows back into the combustion chamber. The fluid flowing out of the combustion chamber thus recirculates.
It is useful that the recirculation opening is constructed as a nozzle, whereby the nozzle merges advantageously with the fluid supply device and/or the antechamber. The nozzle preferably has a constant cross-section, so that the speed of the fluid flowing from the combustion chamber is neither accelerated nor delayed significantly. By means of this nozzle the fluid flowing from the combustion chamber can be specifically added to the flow in the fluid supply device and/or the antechamber. This means that in particular the inflow direction of the fluid flowing from the combustion chamber as well as the point of the merging are freely selectable.
If the recirculation opening first merges with a volume, and then merges indirectly via this volume with the fluid supply device and/or the antechamber, then the point of merging of the interposed volume with the fluid supply device and/or the antechamber are generally to be considered as a merging point of the recirculation opening with the fluid supply device and/or the antechamber also, if this is not separately distinguished.
The recirculation opening is preferably designed so that the narrowest cross-section of the recirculation opening is clearly larger than the narrowest cross-section of a corresponding Helmholtz resonator. A corresponding Helmholtz resonator is specified by the natural acoustic frequency of the combustor, and thus by the rated frequency of the Helmholtz as well as the required attenuation performance. It is especially preferred that the narrowest cross-section of the recirculation opening has a cross-section area that corresponds approximately to ten times the cross-section area of the corresponding Helmholtz resonator. This larger cross-section surface of the recirculation opening in comparison to the Helmholtz resonator is particularly advantageous from the viewpoint of the broadest possible range of action in relation to the oscillation frequencies and oscillation amplitudes to be attenuated. In contrast to a Helmholtz resonator, the sound attenuator suggested here does not provide a resonant sound attenuation. Therefore, the open attenuator cross-section must be greater by a magnitude of about one for the same attenuation performance.
The flow of a real fluid through the combustor principally is subject to loss. The fluid flowing into the combustion chamber thus has a lower total pressure than the fluid in the fluid supply device or in the antechamber. If, due to a static pressure drop, fluid flows through the recirculation opening from the combustion chamber into the fluid supply device and/or the antechamber, the fluid flowing from the combustion chamber thus has a lower total pressure than the fluid in the fluid supply device and/or the antechamber. As a result the mean total pressure in the fluid supply device and/or the antechamber drops downstream from the merging point of the recirculation opening if fluid flows out of the combustion chamber. It is useful that at least one injector is arranged in the combustor in such a way that it merges in an area downstream from the recirculation opening into the fluid supply device and/or the antechamber. Additional fluid can be added to the flow via this injector. The function of the injector is to at least compensate the total pressure drop of the flow across the burner, i.e. the total pressure drop of the flow between the merging point of the recirculation opening with the fluid supply device and/or the antechamber and the corresponding level in the combustion chamber. Additionally the fluid additionally supplied by the injector is advantageously added to the flow at a flow direction adapted to the surrounding fluid flow. It is useful that the injector is constructed as a nozzle with a tapered cross-section. As a result of the additionally added fluid, the mean total pressure of the fluid in the fluid supply device and/or the antechamber rises particularly downstream from the merging point of the injector. As a result a stable rise in pressure that just compensates the pressure drop across the burner occurs in the suction branch of the injector.
It is particularly useful that both the fluid supply device and the injector are supplied from one and the same fluid reservoir, Preferably the free ends of the fluid supply device and of the injector are connected with the fluid reservoir for this purpose.
It is furthermore advantageous that the combustion chamber has the largest possible attenuation volume. The attenuation volume hereby can be constructed as an attenuation chamber. The attenuation volume is arranged so that at least part of the fluid flowing out of the combustion chamber through the recirculation opening flows into the attenuation volume. It is also useful that the attenuation volume is connected to the fluid supply device and/or the antechamber. Compared to the primary zone of the combustion chamber, the attenuation volume has an approximately equal or larger volume. The primary zone is hereby the area of the combustion chamber in which the primary combustion takes place. It was found that the combination of a recirculation opening with an attenuation volume in the form of a buffer volume results in an especially effective oscillation attenuation, particularly for a compressible fluid.
The attenuation volume, in particular the inflow and outflow to the attenuation volume, is preferably designed so that the fluid in the attenuation volume in comparison to the fluid in the combustion chamber has a compensated static pressure at a base load, and a slightly lower static pressure at a full load. With a base load, this results either in no or only a very small flow through the recirculation openings into the attenuation volume. With a full load, the slight overpressure in the combustion chamber results in a continuous outflow of fluid from the combustion chamber through the recirculation opening. By using such a design it is ensured that no fluid will flow through the recirculation opening into the combustion chamber at a full load. An inflow of fluid through the recirculation opening into the combustion chamber would result in a higher emission of noxious substances from the combustion chamber. If no attenuation volume has been provided, it is useful that the area where the recirculation opening merges with the fluid supply device and/or the antechamber is designed so that the fluid in the area of the merging point, when compared to the fluid in the combustion chamber, has a compensated static pressure at a base load, and has a slightly lower static pressure at a full load.
It is furthermore useful that in addition cooler fluid, for example from the fluid supply device and/or the antechamber, flows into the attenuation volume. This prevents too high temperatures from occurring in the attenuation volume.
In a particularly useful design, the attenuation volume has a variable volume size. This makes it possible to vary and optimize the attenuation characteristics of the attenuation volume in a simple manner.
The fluid supply device is advantageously constructed as a Venturi nozzle in the area where it merges with the recirculation opening. The narrowest cross-section of the Venturi nozzle is preferably located directly near the site where the recirculation opening merges. If an attenuation volume is provided, the Venturi nozzle is advantageously arranged in the area where the attenuation volume merges with the fluid supply device, and the narrowest cross-section of the Venturi nozzle is located preferably directly at the point where the attenuation volume merges with the fluid supply device. In particular, by providing a Venturi nozzle, the part of the fluid mass flux through the fluid supply device can be increased in relation to the fluid mass flux through the injector. It is useful that this reduction of the fluid mass flux through the injector is accomplished in a simple manner by reducing the flow cross-section of the injector. As a result of providing the Venturi nozzle, a markedly reduced static pressure of the fluid flow in the fluid supply device occurs in the area of the narrowest cross-section. With the preferred arrangement of the narrowest cross-section of the Venturi nozzle in the immediate area of the merging point of the recirculation opening or attenuation volume into the fluid supply device, the static pressure occurring here corresponds approximately to the static pressure in the combustion chamber. Since at the same time the flow speed of the fluid in the combustion chamber is clearly lower, this therefore results in a markedly lower total pressure of the fluid in the combustion chamber than in the fluid supply device and/or antechamber. Because of the total pressure drop of the flow above the combustor, a stable and directed flow of the fluid in the combustor is substantially ensured even without or only with a small quantity of fluid additionally supplied via an injector. Furthermore, an increased pressure loss in the combustor occurs as a result of providing the Venturi nozzle.